1. Field of the Invention
This invention relates to a process for producing a liquid-container material. More particularly, it is concerned with a process for producing a liquid-container material mainly made of paper and having high gas barrier properties without use of a metal foil.
2. Description of the Related Art
As a liquid-container material mainly made of paper, it has been conventional to use a material comprising a laminate comprising i) a paper substrate comprised of a laminate of paper and polyethylene resin and ii) a polyethylene terephthalate film (PET film) coming in contact with contents and capable of retaining a shape when the material has been formed into a container. On the surface of the PET film coming in contact with contents, a polyethylene resin layer (a sealing layer) is further formed to impart water resistance or waterproofing properties and also impart thermoadhesive properties to the material. To such a liquid-container material, aluminum foil of about 9 .mu.m thick is commonly also laminated as a gas barrier layer for preventing contents from undergoing oxidation and deterioration.
In recent years, it has become problematic tht the polyethylene resin in the sealing layer releases a low-molecular substance into contents or inversely adsorbs or absorbs aroma components present in the contents to damage flavor of the contents. To overcome this problem, a proposal is made on a liquid container in which a non-adsorptive resin such as a polyester resin that does not adsorb or absorb the aroma components and also does not release a low-molecular substance is used as an inner surface layer (for example, Japanese Patent Applications Laid-open No. 57-163654 and No. 1-139336).
Liquid containers of this type make use of a plastic film, and it is difficult to reuse them in an industrial scale after they have been once used. Hence, it is common for them to be put to thermal disposal after their use. There, however, is another problem of a difficulty in the thermal disposal in the case where aluminum foil is used as the gas barrier layer as in the above. For these reasons, in some areas in Europe, restrictions are placed on use of a metal foil such as aluminum foil or the like in liquid containers.
Some contents to be put in liquid containers require microwave heating. This has brought about the problem that no microwave heating can be applied when the metal foil such as aluminum foil is used.
To overcome this problem, it is also proposed not to use the metal foil as the gas barrier layer but to use as a substitute therefor a resin film having excellent gas barrier properties as exemplified by an ethylene-vinyl alcohol copolymer or polyvinylidene chloride. Such a resin film, however, has very poor gas barrier properties compared with aluminum foil and also has a high permeability. Under existing circumstances, use of such a resin film makes it difficult to achieve oxygen barrier properties and moisture barrier properties comparable to those of metal foil, and a film with a thickness feasible for practical use (30 .mu.m or less) can give no sufficient oxygen barrier properties and moisture barrier properties.
As a different means, a technique that can achieve satisfactory gas barrier properties without use of metal foil is proposed, which is a technique in which a microwave permeable sheet mainly made of a plastic film on which a silicon oxide thin-film layer has been formed is used in place of the metal foil conventionally used in liquid-container materials having a paper substrate (Japanese Utility Model Application Laid-open No. 1-96879). This technique can accomplish sufficient oxygen barrier properties and moisture barrier properties.
For reference, comparative data of oxygen barrier properties of aluminum foil (Al-foil), an ethylene-vinyl alcohol copolymer (EVOH), polyvinylidene chloride (PVDC)-coated PET and silicon oxide (SiO) thin-film which is formed on PET are shown in Table 1 below.
TABLE 1 ______________________________________ Oxygen barrier properties Thickness (cc/m.sup.2 .multidot.24 hrs.multidot.atm) ______________________________________ Al foil 9 .mu.m substantially 0 EVOH 12 .mu.m 2 to 10 cc PET/PVDC 15 .mu.m 7 to 15 cc PET(12 .mu.m)/SiO 1,000 .ANG. &lt;1 cc ______________________________________
As is clear from the above comparative data, use of a silicon oxide thin film as a gas barrier layer of a liquid-container material can be considered to give a liquid-container material capable of eliminating the peculiar problems involved when metal foil is used. Formation of an inner surface layer using a non-absorptive resin such as a polyester resin can also be considered to give a liquid-container material that does not damage flavor of contents.
In the manufacture of such liquid-container materials, methods by which a paper substrate or a plastic sheet substrate and a plastic film are laminated are known to include the dry lamination in which a plastic film previously so formed into a film is laminated to the substrate via an adhesive, the heat lamination in which the plastic film is laminated to the substrate under application of heat to the both, and the extrusion lamination in which a thermoadhesive resin is melt-extruded into a film onto the paper substrate or plastic sheet substrate and is laminated to the substrate while being formed into the film.
Incidentally, the thin-film layer of an inorganic compound such as silicon oxide has commonly so poor a flexibility that it can not follow the expansion and contraction of the substrate sheet when the substrate sheet having the thin-film layer of an inorganic compound has undergone expansion and contraction due to heat, to cause cracks. This results in a serious lowering of the gas barrier properties inherent in the thin-film layer of an inorganic compound.
Hence, when liquid-container materials having the thin-film layer of an inorganic compound are produced, the method in which heat is applied to the substrate sheet having the thin-film layer of an inorganic compound can not be employed as the means of lamination. This necessarily follows that the dry lamination must be employed. The lamination carried out by the dry lamination, however, is cost-disadvantageous because of use of an adhesive in a large quantity. Moreover, it requires use of a film with a smooth surface as a laminating film, which is produced by blown-film extrusion, so that the blocking that lowers operability may occur when laminated sheets are wound up during the manufacture. In addition, the finished products (blank sheets for liquid containers) have smooth surfaces and hence the blocking may occur between the blank sheets. This brings about disadvantages such that automatic feeding to the operation for making up box containers can not be constantly carried out.